Xi Ursae Majoris

Xi Ursae Majoris (ξ Ursae Majoris, abbreviated Xi UMa, ξ UMa), also named Alula Australis /æˈluːlə ɔːˈstreɪlɪs/,[10][11] is a star system in the constellation of Ursa Major. On May 2, 1780, Sir William Herschel discovered that this was a binary star system, making it the first such system ever discovered. It was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828. It is also a variable star with a small amplitude. Xi Ursae Majoris is found in the left hind paw of the Great Bear.[12]

Xi Ursae Majoris

The red circle shows the location of Xi Ursae Majoris in Ursa Major
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
ξ UMa A
Right ascension  11h 18m 10.902s[1]
Declination +31° 31′ 44.98″[1]
Apparent magnitude (V) 4.264[1]
ξ UMa B
Right ascension  11h 18m 10.950s[1]
Declination +31° 31′ 45.74″[1]
Apparent magnitude (V) 4.729[1]
Characteristics
Spectral type F8.5:V / G2V[2]
U−B color index 0.04[3]
B−V color index 0.59[3]
Variable type RS CVn[4]
Astrometry
Radial velocity (Rv)−18.2 ± 2.7[5] km/s
Proper motion (μ) RA: −453.7[6] mas/yr
Dec.: −591.4[6] mas/yr
Parallax (π)113.2 ± 4.6[7] mas
Distance29 ± 1 ly
(8.8 ± 0.4 pc)
ξ UMa Aa
Absolute magnitude (MV)4.66[note 1]
Absolute bolometric
magnitude (Mbol)		4.54 ± 0.06[8]
ξ UMa Ba
Absolute magnitude (MV)5.16[note 2]
Absolute bolometric
magnitude (Mbol)		5.00 ± 0.06[8]
Orbit[9]
Primaryξ UMa A
Companionξ UMa B
Period (P)59.878 yr
Semi-major axis (a)2.536″
Eccentricity (e)0.398
Inclination (i)127.94°
Longitude of the node (Ω)101.85 (ascending)°
Periastron epoch (T)1935.195
Orbit[9]
Primaryξ UMa B
Period (P)1.832 yr
Semi-major axis (a)0.057″
Eccentricity (e)0.53
Inclination (i)94.9°
Details[8]
ξ UMa Aa
Mass0.97 M☉
Radius1.02 ± 0.04 R☉
Luminosity1.21 L☉
Surface gravity (log g)4.39 ± 0.10 cgs
Temperature6005 ± 80 K
Rotational velocity (v sin i)1.0 ± 1.0 km/s
ξ UMa Ab
Mass0.38 ± 0.02 M☉
Radius0.32 R☉
Temperature∼3700[note 3] K
ξ UMa Ba
Mass0.86 M☉
Radius0.92 ± 0.04 R☉
Luminosity0.79 L☉
Surface gravity (log g)4.46 ± 0.10 cgs
Temperature5692 ± 90 K
Metallicity [Fe/H]−0.35 ± 0.08 dex
Rotational velocity (v sin i)3.0 ± 1.0 km/s
ξ UMa Bb
Mass0.14+0.05
−0.09 M☉
Other designations
Alula Australis, ξ Ursae Majoris, ξ UMa, Xi UMa, 53 Ursae Majoris, BD+32°2132, CCDM J11182+3132, GC 15537, HIP 55203, IDS 11128+3206, SAO 62484, WDS J11182+3132
A: HD 98231, HR 4375
B: HD 98230, HR 4374
Database references
SIMBADξ UMa
ξ UMa AB
ξ UMa A
ξ UMa B
ξ UMa Bb
WISE J1118+3125

Stellar system

The two components are yellow main sequence stars. The brighter component (designated Xi Ursae Majoris A), has a mean apparent magnitude of +4.41. The companion star (Xi Ursae Majoris B) has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years, and they are currently separated by 1.2 arcseconds, or at least 10 AU.

Orbit of Xi Ursae Majoris

Each component of this double star is itself a single-lined spectroscopic binary. The orbit of the A pair has been determined from spectroscopy and speckle interferometry, giving a period of 669 days and an eccentricity of 0.53.[9] B's binary companion (Xi Ursae Majoris Bb)has not been detected visually or spectroscopically, but the radial velocity variations of the spectral lines show a circular orbit with a period of 3.98 days.[13] The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably red dwarfs, Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.[14]

In 2012 Wright et al. discovered the fifth component and the second brown dwarf (if Bb is also a brown dwarf) of the system using Wide-field Infrared Survey Explorer (WISE) data—a T8.5 brown dwarf WISE J111838.70+312537.9 with angular separation 8.5 arc-min, and the projected physical separation about 4000 AU.[15]

Variable star

ξ Ursae Majoris is classified as an RS Canum Venaticorum variable and its brightness varies by 0.01 magnitudes.[16] Component B is believed to be the variable star, showing characteristic emission lines in its spectrum that are not present for component A.[17]

Nomenclature
Double star Xi Ursae Majoris.(θ):156.6° (ρ):2.1 arc"

ξ Ursae Majoris (Latinised to Xi Ursae Majoris) is the star's Bayer designation.

It also bore the traditional names Alula Australis[18] (and erroneously Alula Australe[19]). Alula (shared with Nu Ursae Majoris) comes from the Arabic phrase Al Ḳafzah al Ūla 'the First Spring'[20] and Australis is Latin for 'the south side'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[21] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[22] included a table of the first two batches of names approved by the WGSN; which included Alula Australis for this star.

In Chinese, 三台 (Sān Tái), meaning Three Steps, refers to an asterism consisting of Xi Ursae Majoris, Iota Ursae Majoris, Kappa Ursae Majoris, Lambda Ursae Majoris, Mu Ursae Majoris and Nu Ursae Majoris. Consequently, the Chinese name for Xi Ursae Majoris itself is 下台二 (Xià Tái èr, English: Star of Second Lower Step).[23]

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See also

References
  1. Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
  2. Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  3. Nicolet, B (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics. 34: 1. Bibcode:1978A&AS...34....1N.
  4. Dempsey, Robert C; Linsky, Jeffrey L; Fleming, Thomas A; Schmitt, J. H. M. M (1993). "The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems". Astrophysical Journal Supplement Series. 86: 599. Bibcode:1993ApJS...86..599D. doi:10.1086/191791.
  5. Nordström, B.; et al. (2004). "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ~14000 F and G dwarfs". Astronomy and Astrophysics. 418 (3): 989–1019. arXiv:astro-ph/0405198. Bibcode:2004A&A...418..989N. doi:10.1051/0004-6361:20035959.
  6. Zacharias, N. (2012). "The fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)". VizieR On-line Data Catalog. 1322. Bibcode:2012yCat.1322....0Z.
  7. Karataş, Y.; Bilir, S.; Eker, Z.; Demircan, O. (April 2004), "Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution", Monthly Notices of the Royal Astronomical Society, 349 (3): 1069–1092, arXiv:astro-ph/0404219, Bibcode:2004MNRAS.349.1069K, doi:10.1111/j.1365-2966.2004.07588.x
  8. Fuhrmann, Klaus (2008). "Nearby stars of the Galactic disc and halo - IV". Monthly Notices of the Royal Astronomical Society. 384 (1): 173–224. Bibcode:2008MNRAS.384..173F. doi:10.1111/j.1365-2966.2007.12671.x.
  9. Mason, Brian D.; McAlister, Harold A.; Hartkopf, William I.; Shara, M. M.; Shara, M. M. (January 1995), "Binary star orbits from speckle interferometry. 7: The multiple system XI Ursae Majoris", The Astronomical Journal, 109 (1669): 332–340, Bibcode:1995AJ....109..332M, doi:10.1086/117277
  10. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  11. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. Griffin, R. F. (1998). "Spectroscopic binary orbits from photoelectric radial velocities. Paper 142: Xi Ursae Majoris". The Observatory. 118: 273. Bibcode:1998Obs...118..273G.
  13. Berman, Louis (1931). "The spectroscopic orbit of the fainter component in the system [xi] Ursae Majoris". Lick Observatory Bulletins. 15: 109–116. Bibcode:1931LicOB..15..109B. doi:10.5479/ADS/bib/1931LicOB.15.109B.
  14. Jim Kaler. "Alula Australis". Retrieved 2019-05-20.
  15. Wright, Edward L.; Skrutskie, M. F.; Kirkpatrick, J. Davy; Gelino, Christopher R.; Griffith, Roger L.; Marsh, Kenneth A.; Jarrett, Tom; Nelson, M. J.; Borish, H. J.; Mace, Gregory; Mainzer, Amanda K.; Eisenhardt, Peter R.; McLean, Ian S.; Tobin, John J.; Cushing, Michael C. (2012). "A T8.5 Brown Dwarf Member of the Xi Ursae Majoris System". The Astronomical Journal. 145 (3): 84. arXiv:1203.5764v1. Bibcode:2013AJ....145...84W. doi:10.1088/0004-6256/145/3/84.
  16. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  17. Strassmeier, Klaus G; Hall, Douglas S; Boyd, Louis J; Genet, Russell M (1989). "Photometric variability in chromospherically active stars. III - the binary stars". Astrophysical Journal Supplement Series. 69: 141. Bibcode:1989ApJS...69..141S. doi:10.1086/191310.
  18. Piazzi, G. (1814). The Palermo Catalogue. Palermo.
  19. Antonín Bečvář (1951). Atlas of the heavens: Atlas coeli. Skalnaté pleso II. Katalog 1950.0. Prírodovedecké vydavatelstvi.
  20. Richard Hinckley Allen. "Star Names — Their Lore and Meaning – Ursa Major, the Greater Bear". Retrieved 2019-05-20.
  21. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  22. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  23. (in Chinese) (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 21 日

Note
  1. Calculated from the bolometric magnitude 4.54 ± 0.06 and the bolometric correction −0.12 ± 0.05 using the formula: BC = Mbol − MV
  2. Calculated from the bolometric magnitude 5.00 ± 0.06 and the bolometric correction −0.16 ± 0.05 using the formula: BC = Mbol − MV
  3. This estimates assumes that ξ UMa Ab is a red dwarf.
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